1. Field of the Invention
The present invention relates to a lithographic apparatus comprising a first control system to control a first physical quantity in the lithographic apparatus, and a device manufacturing a method comprising transferring a pattern from a patterning device onto a substrate.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, control systems are commonly used to control a variety of physical quantities, such as positions, speeds, accelerations, etc. of moving parts, dosages of irradiation, temperatures, gas flows, and many other physical quantities. Thereto, a plurality of control systems are known. It is known, for example, to make use of feedback control systems, feedforward control systems or combined feedback/feedforward control systems. A technique that has been applied in the past years is known under the name iterative learning control (ILC). In iterative learning control, use may be made of a table which is filled with a time series of values. The table may be triggered by an event, upon which the time series of values is provided as a signal, such as a feedforward signal. This process may be made iterative by monitoring the to be controlled value during and/or after the triggering of the table. From the to be controlled quantity, or from any other suitable quantity, such as an error signal, new values for the time series of the table, may now be determined. This process may be repeated a plurality of times, thereby read out to provide an iterative process in which the control system, by monitoring a behavior of the control system itself, or by monitoring a behavior of the output quantity, of the error signal or of any other suitable signal, iteratively determines a new time series for the table, thereby iteratively attempting to fill the table with a time series to provide an optimum response of the control system.
Despite the many benefits of present feedback and feedforward control systems, and the many benefits of iterative learning control, several problems come forward in the current control systems of lithographic apparatus. A first aspect of such problems is that iterative learning control may provide an adequate solution only for repeating, substantially identical situations. For example, in case that a disturbance shows a change as a function of any variable, thereby a different amount of correction by the control system may be required to cancel an effect of such a disturbance as good as possible. Iterative learning control will in such a situation show shortcomings, as the time series stored in the table will normally have been optimized taking a certain amount of disturbance in consideration. An attempt to cope with this situation making use of iterative learning control, is to provide a plurality of time series in a plurality of tables. Depending on a situation, a most suitable one of the tables may be chosen, thereby providing for a better cancellation of the effects of the disturbance, however at an expense of increased complexity. Furthermore, each of the tables requires an iterative process to determine appropriate time series, increasing set-up times, calibration times, etc. Above, even in the situation of a plurality of time series, an optimum cancellation of the effects of the disturbance may only be achieved for situation which match the circumstances under which the particular time series has been determined. Deviations, such as a change in the amount of disturbance due to any reason, will not be taken in account here. An example of the situation described here is a two-dimensional control of a stage (such as a wafer stage, reticle stage, etc.). The stage may commonly be provided with a plurality of control systems, each to control a positional quantity (e.g. a position, a velocity, acceleration, etc.) in a single dimension. In case that a movement in one direction is made, such movement may cause a disturbance on another one of the dimensions. The amount of disturbance (such as cross talk) may vary depending on a position of the stage, thereby making a use of iterative learning control inappropriate to provide e.g. a suitable feedforward signal to compensate for the disturbance.